Wall-flow honeycomb filters are typically used to remove carbonaceous solid particulates from diesel engine exhausts. The honeycomb filter is typically extruded from ceramic precursors mixed with pore forming material. The pore forming material is burned out when the ceramic precursors are fired to produce the hardened ceramic body. FIG. 1 shows a typical wall-flow honeycomb filter 100 having an inlet end face 102, an outlet end face 104, and an array of interconnecting porous walls 106 extending longitudinally from the inlet end face 102 to the outlet end face 104. The interconnecting porous walls 106 define a grid of inlet cells 108 and outlet cells 110. Plugs 112 are inserted at ends of the outlet cells 110 where the outlet cells 110 adjoin the inlet end face 102. Plugs (invisible in the drawing) are also inserted in ends of the inlet cells 108 where the inlet cells 108 adjoin the outlet end face 104. Thus, the outlet cells 110 are open where they adjoin the outlet end face 104, and the inlet cells 110 are open where they adjoin the outlet end face 102.
In a typical cell structure, each inlet cell 108 is bordered on all sides by outlet cells 110 and vice versa. The cells 108, 110 may have a square cross-section as shown. Other cell geometries such as triangle and hexagon are also known. Honeycomb filters having cellular densities between about 10 and 300 cells/in2 (about 1.5 to 46.5 cells/cm2), more typically between about 100 and 200 cells/in2 (about 15.5 to 31 cells/cm2), are considered useful to provide sufficient thin wall surface area in a compact structure. Wall thickness can vary upwards from the minimum dimension providing structural integrity of about 0.002 in. (about 0.05 mm), but are generally less than about 0.060 in. (1.5 mm) to minimize filter volume. A range of between about 0.010 and 0.030 in (about 0.25 and 0.76 mm), e.g., 0.019 in., is most often selected for these materials at the preferred cellular densities.
The honeycomb filter 100 may be installed in a housing, which may then be inserted into the exhaust system of a vehicle equipped with a diesel engine. In operation, diesel exhaust directed at the inlet face 102 of the honeycomb filter 100 flows into the inlet cells 108. The interconnected porous walls 106 are provided with an internal interconnected open porosity that allows the exhaust to pass from the inlet cells 108 to the outlet cells 110 while restraining a desired portion of the solid particulates in the exhaust. The filtered exhaust exits the filter through the outlet cells 110.
Filtration efficiencies up to and in excess of 90% by weight of the diesel exhaust particulates can be achieved with honeycomb filters such as described above. However, the filtration efficiency achievable can be dramatically reduced if there are leaks in the honeycomb filter due to defects, such as holes and cracks, in the interconnecting porous walls and plugs in the filter. Thus in the production of honeycomb filters for diesel particulate filtration, it is customary to test the honeycomb filters for leaks. Defective cells producing leaks are identified and repaired. The test may be repeated until it is ascertained that there are no leaks from the honeycomb filter. The test may be performed while the honeycomb structure is still green or after firing the honeycomb structure. In general, it is easier to repair defects while the honeycomb structure is still green.
One prior-art method for identifying defective cells in a plugged honeycomb filter involves taping a clear film to one end of the honeycomb structure and pouring graphite into the opposite end of the honeycomb structure while rotating the honeycomb structure about two axes. Defective cells having voids within their walls or plugs allow the graphite particles to pass through and are detected by presence of the graphite particles on the clear film. Variations of this method include replacing the graphite particles with other particles, such as micro glass and plastic beads.
Another prior-art method for identifying defective cells in a plugged honeycomb filter is disclosed in U.S. Pat. No. 5,102,434 (Hijikata et al.). In this method, a gas containing solid particulates, such as carbon soot, is flowed under pressure into one end of the honeycomb structure. A gas-permeable screen is placed adjacent the opposite end of the honeycomb structure to collect solid particulates from the gas flowing out of the honeycomb structure. The screen is inspected for patterns differing from the defect-free structure.
The methods described above require fired plugged honeycomb structures and do not reliably detect defects in cases where the solid particulates are too big to flow through the defects. In cases where graphite particles are used for testing, small amounts of graphite particles remain inside the honeycomb structure after testing, which can interfere with the downstream processing of the honeycomb structure, such as catalyst coating process. Further, additional steps are required to clean and remove the solid particulates used for testing from the filter.
Another prior-art method for identifying defective cells in a plugged honeycomb filter involves securing a heat sensitive film (liquid crystal) to one end of a honeycomb filter. The heat sensitive film is initially heated. Cold air is blown from the opposite end of the filter to the film. The air that passes uninhibited through the voids and cracks within the walls of the filter cools the films at the location of the defective cells. This method is suitable for identifying defective cells in green plugged honeycomb filter.